This invention relates to stereo camera systems in which images obtained by two or more spaced apart cameras are used to obtain three-dimensional information about objects in the fields of view of the cameras. The invention has particular application to stereo camera systems for use in high accuracy computer vision applications.
Stereo camera systems are used to acquire three-dimensional information about objects. Stereo camera systems are used, for example, in computer vision systems, 3-dimensional tracking applications, object dimensioning applications, object tracking applications, and so on. Typical stereo camera systems include at least two, and typically three or more, electronic cameras which are mounted at spaced apart locations. The electronic cameras have overlapping fields of view. A computer connected to receive images from each of the cameras can compare the images to derive three-dimensional information about objects in the field of view. Information such as the distances to the objects and the sizes, dimensions and orientations of the objects can be determined by triangulation.
A stereo camera system is typically calibrated by placing a known object in the field of view of the cameras. Computer software which receives images from the cameras can determine the precise relationship between the cameras from the images of the known object and also compensate for distortions introduced by the lenses. After the stereo camera system has been calibrated then the computer can be used to obtain information about objects whose positions or configurations are not known.
Currently available stereo camera systems use small cameras which have arrays of light sensing elements such as charge coupled devices (xe2x80x9cCCDsxe2x80x9d), CMOS sensors or the like. A typical camera, as is used in a typical stereo camera system, comprises a circuit board on which a light sensing array is mounted. A lens is supported above the light sensing array by a lens holder. The entire camera is mounted by the circuit board to a suitable support in a desired position relative to other cameras in the stereo camera system.
A problem with such existing stereo camera systems is that the calibration of the systems can degrade over time. For a stereo camera system to remain perfectly calibrated the fields of view of the cameras must not move relative to one another. In a conventional stereo camera system there is potential for movement in the mounting between the lens holders and the circuit boards, in the mounting between the circuit boards and the substrate or frame to which the circuit boards are attached and there is potential for movement of the frame itself, The multiple interfaces between components in a conventional stereo camera system make it highly likely that vibration, shocks or the like will cause the field of view of one or more cameras in a stereo camera system to shift over time. If this happens then calibration will be lost.
This problem has not been recognized in the prior art because prior art stereo camera systems have either been laboratory type systems not subjected to shocks or vibrations or have been used in situations where highly accurate calibration has not been regarded as being critical. The inherent susceptibility of current stereo camera systems to losing calibration has therefore not been recognized as a particular problem.
Tocher, U.S. Pat. No. 5,483,336 discloses a stereoscopic optical rangefinder system. The system includes two arrays of photosensitive elements which each have associated focussing optics. A separate optical system corrects for relative motions of the arrays and the focussing optics which might be caused by mechanical shock or thermal vibrations. The Tocher system adds expense and complication. It is not clear whether the Tocher system can compensate for the effects of distorted lenses or rotations of light sensing arrays relative to one another.
There is a need for stereo camera systems having two or more cameras which can be used for extended periods in environments where they are exposed to vibration and or shock without losing calibration.
This invention provides a stereo camera system in which a rigid mounting member, which preferably comprises a metal plate, directly supports both a lens assembly and a light sensing assembly for each of two or more cameras. The mounting member holds the lens assemblies and light sensing assemblies rigidly relative to one another.
One aspect of the invention provides a stereo camera system comprising rigid mounting member having a front face and a rear face; a plurality of lens assemblies rigidly affixed to the mounting member, the lens assemblies each comprising a lens; and, a plurality of light sensing assemblies rigidly affixed to the mounting member. The mounting member is penetrated by a plurality of apertures which extend between the front and rear faces. The apertures provide paths for carrying light which is focussed by the lens assemblies onto the light sensing assemblies. Each of the lenses is located to focus light through one of the apertures onto one of the light sensing assemblies.